1. Field of the Invention
The present invention pertains to the field of optical communication networks. More particularly, the present invention pertains to methods and apparatus for introducing narrowcast transmission signals into a fiber optic broadcast network.
2. Prior Art Systems and Methods
Communication networks exist in a wide variety of types and sizes and are used for a wide variety of applications. Increasing use of fiber optic transport technology is being made for relatively large communication networks, such as telecommunications distribution networks.
Fiber optic communication systems, such as those currently used in telecommunications distribution networks, typically require amplification of the signal light to compensate for optical power (Db) loses which occur as the signal light travels over long distances, for example over 20 km. This amplification also serves to compensate for losses due to splitting the signal light between different branches of the network. In the past, electronic regeneration, which required photoelectric conversion, was used. More modern systems, however, employ optical amplification systems to amplify the signal light without requiring photoelectric conversion.
Modern fiber optic communication systems utilize optical amplification systems having an optical fiber whose core is doped with a rare-earth, such as erbium (Er). Rare-earth-doped fiber amplification systems, such as known erbium-doped fiber amplifier ("EDFA") systems, are well known in the art. In particular, EDFA technology offers high power gain and operational efficiency in the 1550 nm transmission wavelength band, which nominally covers a range of actual wavelengths from approximately 1530 nm to approximately 1560 nm. Since the lowest transmission power loss wavelength for optical fibers is in this 1550 nm range, rare-earth-doped fiber amplification systems, such as EDFA systems, which provide high power gain and operational efficiency in this bandwidth, are desirable for use in fiber optic communication systems. EDFA technology is also advantageous in that it is transparent to the transmission format of the signal light.
Typical EDFA systems amplify a signal light by passing the signal light through an erbium-doped optical fiber while simultaneously "pumping" the erbium-doped fiber with a relatively powerful light having a wavelength approximately equal to the absorption wavelength of erbium ions. In certain applications, it has been found advantageous to provide the pumping light in both the "forward" direction, (i.e., propagating in the same direction the signal light is traveling), and the "backward" direction, (i.e., propagating opposite to the direction that the signal light is traveling), although in other applications it has been found that is only necessary to provide the pumping light in one direction. The amount of amplification to the signal light generated by the EDFA is a function of, among other things, the physical length of the erbium-doped fiber (typically between 5 to 20 m), the wavelength of the pumping signal light(s) and the pump source power (generally greater than 50 mW). While the wavelength of the pumping light(s) for erbium-doped fiber amplifiers may be in the 500 nm, 600 nm, 800 nm, 980 nm, or 1480 nm bands, only the 1480 nm and 980 nm bands are generally used as they provide the most optimal power gain (up to 40 mW) to the signal light.
In a first exemplary known EDFA configuration, inserted in an "in-line" configuration along an incoming transport fiber carrying one or more 1550 nm band optical signals to be amplified, a fiber containing the output light from a 1480 nm forward pumping source is coupled with the transport fiber to the input end of an erbium-doped "gain fiber" by a wave division multiplexer (WDM), which directs both light sources into the gain fiber. The WDM may be an E-Tek dynamics model SWDM-1415-A, or a Gould Electronics model 1480-1550-COW-MX-02X02-01, or the like. A fiber containing the output light from a 980 nm backward pumping source is coupled to the output end of the gain fiber by a second WDM, which directs the 980 nm pumping light into the gain fiber propagating in the opposite direction than the 1480 nm pumping light, and also directs the amplified 1550 nm band signal(s) back into the transport fiber. An 1550 nm band optical isolator, such as a Gould Electronics OPT15015 or the like, is inserted across the transport fiber, after the second WDM, to prevent back reflections emanating from the "downstream" direction in the optical network from passing into the gain fiber, which may adversely affect the operation of the EDFA. The optical isolator also acts to effectively remove "power leakage" representing an "unused" portion of the 1480 nm pumping light, i.e., a portion of the 1480 nm light that is not absorbed by the erbium ions in the gain fiber, which exits the gain fiber along with the amplified 1550 nm band signal(s) and, via the second WDM, propagates into the transport fiber. Because only lightwave bandwidths of approximately 1200 nm to 1600 nm will propagate in typical optical networks, (which is a function of the core diameter of the transmission fiber, as well as signal transmission and reception limitations, respectfully, of the transmission equipment at either end), isolation of any reflected portion of the backward pumped 980 nm lightwave is not generally necessary.
In a second exemplary known EDFA configuration, identical to the first except that a 980 nm light is used for forward pumping and a 1480 nm light is used for backward pumping in the gain fiber, any power leakage in the form of reflected backward pumped 1480 nm light will be removed by the 1550 nm band optical isolator.
It is known to use a 1550 nm band wavelength based "global" optical communication network utilizing rare-earth-doped fiber amplifier technology, such as EDFAs, for transmitting one or more "broadcast" communication signals to a large plurality of end users or "subscribers," where long transmission distances, large subscriber bases and frequent optical splitting necessitate repeated optical signal regeneration in the form of cascaded amplifiers throughout branches of the network. As used herein, a "broadcast" signal refers to any communication signal, preferably in the RF range, i.e., at least 1 Mhz, which is intended to reach all subscribers connected to the communication network. A broadcast signal may contain, by way of example, telecommunications voice or data information, video signals, or the like. For instance, in a cable television transmission network, certain television signals are typically "broadcast" to all subscribers connected to the network.
It is also known to transmit one or more communication signals over smaller, "local" communication networks which generally have few or no amplifiers and have relatively few subscribers compared with the global networks previously discussed. Many optical networks, such as 1300 nm band wavelength based video transport networks, cannot utilize EDFA technology, since the EDFAs will effectively amplify optical signals only in the 1550 nm band; i.e., from approximately 1530 nm to approximately 1560 nm.
Because of the inherent advantage in performing the greatest possible number of transmission applications over a single optical communication network, it is desirable to be able to use global optical broadcast networks, such as a new or existing fiber optic telecommunications distribution network, to transport particular local signals, referred to herein as "narrowcast" signals, over limited portions of the global network. As used herein, a "narrowcast" signal may be identical to a "broadcast" signal in form or content, but is intended to be transmitted over only a limited portion of the global network; i.e., it is a signal that is selectively "narrowcast" to only some of the subscribers of the global network. For instance, a "video on demand" service in a major metropolitan area may wish to offer a limited number of local subscribers of a global telecommunications network that spans an entire metropolitan area the opportunity to view one of several hundred movies via a "narrowcast" transmission from the business location to each of the subscribers over the global telecommunications network. Other common examples of narrowcasting applications, although not an exhaustive sample, include business advertisements directed only to a limited group of local subscribers of a particular global broadcast network, or internal communications signals (e.g., for monitoring and maintenance purposes) sent between different "intelligent" components of a global telecommunications network.
Therefore, it is desirable to be able to transmit narrowcast signals in a 1550 nm band optical broadcast network, including those utilizing rare-earth-doped fiber amplifier technology, such as EDFA technology. Present global fiber optic communications networks do not adequately address the problems associated with a single optical network transmitting both "broadcast" and "narrowcast" signals.